Viruses can be divided into several (arbitrary) classifications based upon the type of nucleic acid that they carry and the mode of expression. Table 1 illustrates one classification scheme based upon “The International Code of Virus Classification and Nomenclature”, and the classification system developed by Dr. David Baltimore (incorporated herein by reference)
TABLE 1Group NameNucleic Acid TypeOrderExamplesDNA virusesDouble strandedOrder:Ex. Enterobacteria phage T4DNA virusesCaudoviralesUnassignedEx. Family AdenoviridaevirusesEx. Family HerpesviridaeEx. Family Polymaviridae (simiam virus40)Ex. Family Poxviridae (Cowpox virus,Variola virus-smallpox)Single strandedUnassignedDNA virusesbacteriophagesUnassignedEx. Family ParvoviridaevirusesRNA virusesDouble strandedUnassignedEx. Family ReoviridaeRNA virusesviruses(+) single-strandedOrder:Ex. Family Coronaviridae (coronavirus,RNA or mRNA-likeNidoviralesSARS)virusesUnassignedEx. Family Flaviviridae (Yellow fevervirusesvirus, West Nile virus, Hepatitis C virus)Ex. Family Picornaviridae (poliovirus,rhino virus, hepatitis A virus)Ex. Togaviridae (Rubella virus)(−) single-strandedOrder:Ex. Family Paramyxoviridae (measlesRNA virusesMononegaviralesvirus, mumps virus)(non-segmentedEx. Family Rhabdoviridae (rabies virus)negative strandedviruses)SegmentedEx. Family Orthomyxoviridae (Influenzanegative strandedviruses)virusesDNA and RNASingle-strandedUnassignedEx. Family Retroviridae (Retroviruses,reverseRNA reversevirusesHIV)transcribingtranscribing virusesvirusesDouble-strandedUnassignedEx. Family Hepadnaviridae (Hepatitis BDNA reversevirusesvirus)transcribing virus
The design and discovery of new antiviral drugs can be directed against either viral or cellular targets. Drugs that inhibit viral proteins are more likely to be virus-specific and are more prone to the development of resistance. Thus, there is an urgent need for broad spectrum antiviral drugs that can be used in mono- or combined therapy to treat the numerous viral diseases caused by DNA viruses, RNA viruses and DNA and RNA reverse transcribing viruses for which limited or no therapeutic options are currently available. The demand for new effective and safe antiviral strategies has become an increasingly important problem to solve in recent years due to the rising prevalence of chronic viral infections such as HIV and hepatitis B and C, the emergence of new viruses or viral strains such as the SARS coronavirus and pathogenic Avian Influenza strains, the ever-present threat of viral pandemics from agents like virulent strains of influenza A, and the potential danger of hemorrhagic fever viruses and eradicated viruses such as variola virus being exploited as bioterrorist weapons.
A limited number of chronic viral diseases, which affect millions of people world-wide (e.g., 40 million individuals are HIV-infected), can be controlled to some degree, but no cures are currently available. As a result, infected individuals provide a reservoir for the respective virus through which naïve individuals become infected, thereby, perpetuating the problem of viral infection of significant human populations with these pathogens. Currently available antiviral drugs, such as the nucleoside reverse transcriptase inhibitors (NRTIs) used to inhibit viral replication of specific pathogenic viruses, have resulted in recognizable improvements in the ability to control infections with these pathogens and to improve the quality and length of life of infected individuals. A partial list of currently available antiviral drugs used to inhibit viral replication of specific pathogenic viruses is provided in Appendix 2 (for eg. the NRTIs). The therapies listed have resulted in recognizable improvements in the ability to control infections from these pathogens and to improve the quality and length of life of infected individuals. However, currently available classes of antiviral agents have limited utilities due to their narrow scope of activities against different viruses and/or problems with significant drug-induced toxicities. In addition, the modes of action of the NRTIs and other drugs in current clinical use predispose to the development of drug resistance through viral mutations. Finally, many currently available drugs have considerable side effects that prevent their wide spread use to achieve treatment or prevention goals.